1. Field of the Invention
The present invention relates to the production of ultrathin layers of amorphous metal oxides and oxide heterostructures for use as dielectric insulators comprising the use of a reactive atomic or molecular beam concurrent oxidation and deposition technique.
2. Brief Description of the Prior Art
There is a need for ultrathin amorphous oxides and oxide heterostructures for use as dielectric insulators. One of the major applications of such oxides are as new gate dielectrics for Si CMOS technology as a replacement for SiO2 based gate dielectrics which become less attractive with the sing dimensions of CMOS scaling. Such new gate dielectrics will need to be insulating with low leakage currents, should have a high dielectric constant, should be robust and inert to the environments of the CMOS process conditions and should be non-reactive with Si. One should be able to make such thin films as heterostructures in multilayer form (with thicknesses at the atomic or molecular dimensions) and should also be able to dope them with other elements in order to enhance dielectric constants as well as to maintain an amorphous structure. In other words, one should be able to deliberately engineer such layers with chemical composition and microstructural modulations at atomic or molecular layer level thicknesses. What the alternative gate dielectric for the future will be is not known at the present, though there are several candidate materials. It is reasonable to expect that such a material will be an oxide. What is needed is a technique for depositing such oxide layers in a pure fashion from low to high temperatures with atomic or molecular layer thickness controls. Such oxides may also be useful in novel device structures requiring processing at low temperatures.
In the past, aluminum oxide films have been deposited by techniques such as conventional sputtering and chemical vapor deposition. Conventional sputtering results in ion damage. In addition, it is difficult to prevent or deliberately control the formation of an intermediate silicon oxide layer. Chemical vapor deposition techniques contain hydrogen environments and therefore can result in the formation of aluminum hydroxides. These hydroxides are unstable above 400xc2x0 C. and convert to oxides, resulting in microstructural changes in the film. The atomic or molecular beam deposition technique described herein bypasses the above-noted problems, and results in the purest, most controllable deposition possible. The present invention provides a unique undoped or doped film grown by the atomic or molecular beam deposition technique.
Other objects and features as well as additional details of the present invention will become apparent from the following detailed description and annexed drawings of the presently preferred embodiments thereof, when considered in conjunction with the drawings.
The present invention demonstrates the production of ultrathin layers of metal oxides and oxide heterostructures for use as dielectric insulators comprising the use of a reactive atomic or molecular beam concurrent oxidation and deposition technique. In accordance with the method, the sample is held in an ultra high vacuum deposition chamber and faces an atomic or molecular beam source of oxygen and elemental evaporation sources. The growth of an oxide is accomplished by evaporating the elemental constituents in the presence of the atomic or molecular oxygen beam. The resultant thin film deposited thereupon is an oxide compound of the elemental evaporants. For example, if the evaporated elements are A, B and C, the deposited thin films will have the chemical composition AxByCdOy. Since highly reactive oxygen can be used as a source material, an elevated temperature is not necessary for oxidation, and the oxide deposition may be carried out to temperatures below room temperature, (less than about 30xc2x0 C. up to about 1000xc2x0 C.). This is an advantage of the technique. Additionally, the same atomic or molecular oxygen source can also be used to oxidize the Si surface for the formation of an interfacial SiO2, if necessary, again at any convenient temperature. The convenient temperature noted and the atomic and molecular oxygen source allow SiO2 layer formation with controllable thickness. Since the evaporants can be varied and the evaporation sources can be shuttered, abrupt multilayered oxides with different chemical compositions may be formed with thicknesses controllable to one monolayer. Finally the technique of the present invention lends itself very easily to the formation of doped oxides, by simple evaporation of the dopant. The process is depicted in FIG. 1.
In summary, the deposition obtained in the present invention occurs by delivery of a metal flux or vapor onto the substrate surface. In the presence of an oxygen beam the metal oxidizes as it is deposited. The process of the present invention is thus one where a layer of metal is oxidized, either thermally or by an oxygen rf beam, and then buried below as a fresh layer of metal is deposited over it, and in turn is oxidized as well. This is accomplished by having a molecular or rf discharge excited oxygen beam turned at all times towards the substrate, with the metal being continuously evaporated at a slow rate in conjunction; or alternatively, the metal and oxygen beams are sequentially shuttered toward the substrate.
The present, invention has taken the concept of a molecular or atomic beam with directionality as it impinges upon the substrate and uses the beam of oxygen for oxidation of an elemental metal as it is deposited upon the substrate.
The method of the present invention provides four benefits: (i) precisely controlled deposition of amorphous multilayers in a clean ultra high vacuum environment; (ii) low temperature deposition leveraged by the ability to use atomic oxygen; (iii) ease of doping films; (iv) the ability to tailor the chemical composition of the upper surface of the silicon substrate prior to deposition by deliberate exposure of the surface to an oxygen beam at any convenient temperature.
The deposition technique of the present invention derives from well known molecular beam epitaxy technique, where single crystal epitaxial films are grown at high temperatures on substrates. Optionally, the atomic oxygen source is a commercial radio frequency source; however, the desired oxide may be formed (grown) using only a molecular oxygen flow. The pure environment insures a carefully controlled interface between the layers without the presence of unintentional contaminants and chemical layers.
Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to currently preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the method and apparatus illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. In addition it is to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended herewith.